Mars today is pretty dry. But billions of years ago, water flowed across the Red Planet. It ran in rivers that carved deep valleys. And it may have even filled a Martian ocean inside what today look like the remains of ancient shorelines.

So where’d all the water go? Some of it is locked up in polar ice caps. But much more may be buried in the ground.

"It’s a good amount of water."

That's Jack Mustard, professor of geological sciences at Brown University, on March 20th at the Lunar and Planetary Science Conference in The Woodlands, Texas.

These hydrated minerals, he says, are all over. And they run pretty deep.“We used a variety of indicators to estimate the depth. One of them is the vast cracks we have in the surface of Mars, such as in Valles Marineris, which can go down to eight kilometers. And we can detect phyllosilicates to that depth, so that tells us at least eight kilometers deep. But we also survey impact craters, which are like drill holes into the surface of Mars. And the bigger the crater, the deeper the drill hole.

“So we’ve identified hydrated minerals in craters as large as 100 kilometers, which gives us a rough estimate of around 10 kilometers depth that we’ve excavated from there. So for those two independent pieces of evidence we estimate at least the top 10 kilometers of the crust have strong signatures of hydrated minerals in them.”

If all the water were liberated from those minerals and spread across the entire planet, it would make a pretty impressive reservoir.

"If we take all the water that we know to exist in the polar caps on Mars, it's around 30 meters of water. So this is probably around three to five times the amount of water that we see currently in the polar caps on Mars."

That amount might not impress deep-diving James Cameron, but it's a significant reservoir by Mars standards. Freed up, it would flood the entire Martian surface under at least 100 meters of water. "It's enough to go swimming in."